The relative in vivo contribution of permissive cell populations to HIV-1 viremia and total virus load during acute and subsequent persistent infections is poorly understood. A better understanding of the contribution of tissue macrophages and CD4+ T cell subpopulations will allow a more complete picture of HIV pathogenesis and potentially stimulate the development of cell-specific antiviral approaches that are likely to decrease side effects that are associated with current antiviral therapies. To date, approaches to understand the impact of the different cellular compartments in vivo have been limited and laborious. The novel approach proposed to be developed here will allow the field to clearly define the contribution of these cells to virus load and cytokine/chemokine changes. In these tissue culture and explant studies, we propose to develop a new and innovative technology to control the tropism of HIV for the purposes of delineating the contribution of specific cell types for HIV infection and pathogenesis and provide proof-of-principal that when used in vivo can provide important insights into impact of the cellular reservoirs of HIV. We will engineer recombinant HIV strains that are restricted in either myeloid- or lymphoid-specific cultures by harnessing cell- specific expression of endogenous miRNAs. Through incorporation of perfect miRNA target sites into the HIV genome, expression of these HIVs are restricted in targeted cellular subtypes, thereby allowing us to ascertain the contribution of these cells by a loss of function phenotype. By restricting HIV replication in a variety of different hematopoietic cellular subsets, we will determine how this impacts both virus and host biology in the context of primary ex vivo infections.